An international team of scientists has uncovered an important part of the global greenhouse gas budget. This new analysis indicates that greenhouse gas uptake by continents is less optimistic than previously thought. The balance between carbon uptake by continents and their emissions of greenhouse gases is important because it indicates how much continents can compensate for human emissions of CO2 to the atmosphere.

Past analyses of carbon and greenhouse gas exchanges on continents have failed to account for the influence of lakes, impoundments, and running water. This study, published today in the journal Science, shows that natural release of the potent greenhouse gas methane from inland waters may be far greater than previously known. By difference, the net absorption of greenhouse gases by natural land environments, such as forests, may therefore be at least 25 % smaller than thought.

This is the conclusion of a study by David Bastviken, Linköping University, Lars Tranvik, Uppsala University, John Downing, Iowa State University, Patrick Crill, Stockholm University, and Alex Enrich-Prast, University Federal of Rio de Janeiro.

The increased greenhouse effect is caused by human emissions of greenhouse gases. Some ecosystems, such as forests can absorb carbon dioxide and act as greenhouse gas sinks, which is important for the greenhouse gas balance and the climate. The role of freshwater environments, integrated into continental environments, has been unclear because of underestimates of the amount of continental water and a shortage of data on greenhouse gas emissions.

Methane emissions from lakes and running water occur naturally and should not be considered an environmental threat. These gas emissions have, however, been difficult to assess and are poorly understood.

- Small methane emissions from the surfaces of water bodies occur continuously, says David Bastviken, but much greater emissions occur suddenly, and with irregular timing, when methane bubbles from the sediment reach the atmosphere. Such fluxes have been difficult to measure.

The authors have summarized methane fluxes from 474 freshwater environments. They have also used updated estimates of the global area of inland waters. Based on these data, they estimated that methane emissions from the freshwaters of the world counter-balance 25 percent of the carbon dioxide absorbed by natural land environments. The large effect of aquatic methane emission is due to the large quantity of gas emission and the stronger greenhouse effect produced by methane molecules compared to carbon dioxide. One implication of this new accounting is that the greenhouse gas sink provided by forests and other land ecosystems is substantially smaller than hitherto believed.

- The terrestrial sink may even be lower than we have calculated, says David Bastviken. Because it is difficult to measure methane bubble fluxes and the global area of freshwaters may still be underestimated, we have probably underestimated the methane emissions.

If we do not properly account for natural greenhouse gas sinks and emissions, we may misunderstand the urgency of reducing anthropogenic emissions.

A smaller continental greenhouse gas sink means that the capacity of natural systems to absorb greenhouse gases is very valuable.

- We have to take great care of the remaining forests and other natural greenhouse gas sinks, because we have already reduced their area through deforestation and other land conversions, David says. An accurate accounting of all components of the continental greenhouse gas budget, including the role of inland waters, will help us evaluate the greenhouse gas uptake by land ecosystems in relation to society’s greenhouse gas emissions.